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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.overview.core.basics"></a><a class="link" href="basics.html" title="Basic Boost.Asio Anatomy">Basic Boost.Asio Anatomy</a>
</h4></div></div></div>
<p>
          Boost.Asio may be used to perform both synchronous and asynchronous operations
          on I/O objects such as sockets. Before using Boost.Asio it may be useful
          to get a conceptual picture of the various parts of Boost.Asio, your program,
          and how they work together.
        </p>
<p>
          As an introductory example, let's consider what happens when you perform
          a connect operation on a socket. We shall start by examining synchronous
          operations.
        </p>
<p>
          <span class="inlinemediaobject"><img src="../../sync_op.png" alt="sync_op"></span>
        </p>
<p>
          <span class="bold"><strong>Your program</strong></span> will have at least one <span class="bold"><strong>I/O execution context</strong></span>, such as an <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span></code> object, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">thread_pool</span></code>
          object, or <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">system_context</span></code>. This <span class="bold"><strong>I/O
          execution context</strong></span> represents <span class="bold"><strong>your program</strong></span>'s
          link to the <span class="bold"><strong>operating system</strong></span>'s I/O services.
        </p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span> <span class="identifier">io_context</span><span class="special">;</span>
</pre>
<p>
          To perform I/O operations <span class="bold"><strong>your program</strong></span>
          will need an <span class="bold"><strong>I/O object</strong></span> such as a TCP
          socket:
        </p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span> <span class="identifier">socket</span><span class="special">(</span><span class="identifier">io_context</span><span class="special">);</span>
</pre>
<p>
          When a synchronous connect operation is performed, the following sequence
          of events occurs:
        </p>
<p>
          1. <span class="bold"><strong>Your program</strong></span> initiates the connect
          operation by calling the <span class="bold"><strong>I/O object</strong></span>:
        </p>
<pre class="programlisting"><span class="identifier">socket</span><span class="special">.</span><span class="identifier">connect</span><span class="special">(</span><span class="identifier">server_endpoint</span><span class="special">);</span>
</pre>
<p>
          2. The <span class="bold"><strong>I/O object</strong></span> forwards the request
          to the <span class="bold"><strong>I/O execution context</strong></span>.
        </p>
<p>
          3. The <span class="bold"><strong>I/O execution context</strong></span> calls on
          the <span class="bold"><strong>operating system</strong></span> to perform the connect
          operation.
        </p>
<p>
          4. The <span class="bold"><strong>operating system</strong></span> returns the result
          of the operation to the <span class="bold"><strong>I/O execution context</strong></span>.
        </p>
<p>
          5. The <span class="bold"><strong>I/O execution context</strong></span> translates
          any error resulting from the operation into an object of type <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span></code>. An <code class="computeroutput"><span class="identifier">error_code</span></code>
          may be compared with specific values, or tested as a boolean (where a
          <code class="computeroutput"><span class="keyword">false</span></code> result means that no
          error occurred). The result is then forwarded back up to the <span class="bold"><strong>I/O object</strong></span>.
        </p>
<p>
          6. The <span class="bold"><strong>I/O object</strong></span> throws an exception
          of type <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">system_error</span></code> if the operation failed.
          If the code to initiate the operation had instead been written as:
        </p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">;</span>
<span class="identifier">socket</span><span class="special">.</span><span class="identifier">connect</span><span class="special">(</span><span class="identifier">server_endpoint</span><span class="special">,</span> <span class="identifier">ec</span><span class="special">);</span>
</pre>
<p>
          then the <code class="computeroutput"><span class="identifier">error_code</span></code> variable
          <code class="computeroutput"><span class="identifier">ec</span></code> would be set to the
          result of the operation, and no exception would be thrown.
        </p>
<p>
          When an asynchronous operation is used, a different sequence of events
          occurs.
        </p>
<p>
          <span class="inlinemediaobject"><img src="../../async_op1.png" alt="async_op1"></span>
        </p>
<p>
          1. <span class="bold"><strong>Your program</strong></span> initiates the connect
          operation by calling the <span class="bold"><strong>I/O object</strong></span>:
        </p>
<pre class="programlisting"><span class="identifier">socket</span><span class="special">.</span><span class="identifier">async_connect</span><span class="special">(</span><span class="identifier">server_endpoint</span><span class="special">,</span> <span class="identifier">your_completion_handler</span><span class="special">);</span>
</pre>
<p>
          where <code class="computeroutput"><span class="identifier">your_completion_handler</span></code>
          is a function or function object with the signature:
        </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">your_completion_handler</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&amp;</span> <span class="identifier">ec</span><span class="special">);</span>
</pre>
<p>
          The exact signature required depends on the asynchronous operation being
          performed. The reference documentation indicates the appropriate form for
          each operation.
        </p>
<p>
          2. The <span class="bold"><strong>I/O object</strong></span> forwards the request
          to the <span class="bold"><strong>I/O execution context</strong></span>.
        </p>
<p>
          3. The <span class="bold"><strong>I/O execution context</strong></span> signals to
          the <span class="bold"><strong>operating system</strong></span> that it should start
          an asynchronous connect.
        </p>
<p>
          Time passes. (In the synchronous case this wait would have been contained
          entirely within the duration of the connect operation.)
        </p>
<p>
          <span class="inlinemediaobject"><img src="../../async_op2.png" alt="async_op2"></span>
        </p>
<p>
          4. The <span class="bold"><strong>operating system</strong></span> indicates that
          the connect operation has completed by placing the result on a queue, ready
          to be picked up by the <span class="bold"><strong>I/O execution context</strong></span>.
        </p>
<p>
          5. When using an <code class="computeroutput"><span class="identifier">io_context</span></code>
          as the <span class="bold"><strong>I/O execution context</strong></span>, <span class="bold"><strong>your program</strong></span> must make a call to <code class="computeroutput"><span class="identifier">io_context</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code> (or to one of the similar <code class="computeroutput"><span class="identifier">io_context</span></code> member functions) in order
          for the result to be retrieved. A call to <code class="computeroutput"><span class="identifier">io_context</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code> blocks while there are unfinished asynchronous
          operations, so you would typically call it as soon as you have started
          your first asynchronous operation.
        </p>
<p>
          6. While inside the call to <code class="computeroutput"><span class="identifier">io_context</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>, the <span class="bold"><strong>I/O execution
          context</strong></span> dequeues the result of the operation, translates it
          into an <code class="computeroutput"><span class="identifier">error_code</span></code>, and
          then passes it to <span class="bold"><strong>your completion handler</strong></span>.
        </p>
<p>
          This is a simplified picture of how Boost.Asio operates. You will want
          to delve further into the documentation if your needs are more advanced,
          such as extending Boost.Asio to perform other types of asynchronous operations.
        </p>
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